In the industrial processing of plant materials, such as potatoes, tobacco or cacao, the fibrous mass obtained, is among the by-products isolated in huge quantities. Similar by-products or waste products are known from many other agriculturally based productions, e.g., sugar beet slices from the production of sugar from sugar beets, citrus peels and citrus pulp from the manufacture of juice and ethereal oils from citrus fruits, and pomace residues from cider production.
These plant by-products are often considered as waste products to be disposed in the most appropriate and most inexpensive way. However, it will be understood that there may be quite obvious advantages by further developing such plant by-products into products of more commercial value.
A common feature of these by-products is that they consist essentially of soluble and insoluble plant fibres, of which about 60-80% are dietary fibres, including three biopolymers: cellulose, hemicellulose and pectin. They are involved in the structure of all plant cell walls, which can be conceived as a cellulose-hemicellulose-pectin network in which pectin, apart from being a structural element, also constitutes the “concrete” imparting rigidity to the plant cells and adhesion function in the space between them. This complex structure in which pectin is attached to the other cell wall components by different types of bonds for instance hydrogen bonds and/or ionic interaction is often termed protopectin. Pectin, per se, can be obtained by controlled, acidic or alkaline hydrolytic extraction of protopectin.
Pectin is a polymer of extreme complexity, composed of a backbone(s) of units of α-D-galacturonic acid attached by α-1,4-glycoside bonds to form long chains of polygalacturonic acid. The homogalacturonic regions are interspersed with rhamnogalacturonic regions with 1,2 linked α-L-rhamnopyranosyl residues bearing different kinds and amounts of neutral sugar side chains. The galacturonic acid units are esterified with methanol to a varying degree and can be partly esterified with acetyl on the secondary hydroxyls. An industrial distinction is thus made between high-methoxylated-pectin having a degree of esterification (DE) of greater than 50% and low-methoxylated-pectin having a degree of esterification of less than 50%. The degree of esterification is defined as the number of methyl-esterified galacturonic acid units expressed as a percentage of the total galacturonic acid units in the pectin molecule.
In pectin from some types of plant material, e.g. potatoes and sugar beets, a varying part of the galacturonic acid units may, in addition, have acetyl groups located on C-2 and C-3 positions, expressed as the degree of acetylation (DAc). DAc is defined, analogous to the degree of esterification, as the number of acetylated galacturonic acid units as a percentage of all galacturonic acid units.
Neutral sugars, such as galactose, glucose, rhamnose, arabinose and xylose, may also be part of the pectin polymer as side-chains to or as members in the polygalacturonic acid chain.
Hemicellulose is a heterogeneous group of polysaccharides containing several kinds of hexose and pentose sugars and, in some cases, residues of uronic acid. These polymers are classified according to the type of sugar residues being dominant and are individually referred to as xylanes, arabinogalactans, glucomannans and so on.
Conventionally, methods for isolating pectin from a plant material are provided, where the plant material is suspended in an acidic solution having a pH around 1-3 at elevated temperature and/or long extraction times, whereby the pectin begin to disintegrate and is being extracted from the plant material. Subsequently the solid phase and the liquid phase may be separated and the pectin may be isolated/purified from the liquid phase.
Thus, U.S. Pat. No. 5,567,462 discloses a method of preparing pecto-cellulosic compositions and pectin from pectin-containing plant raw materials, such as citrus peels, sugar beet pulp, sunflower residues, and pomace residues. The method consists of treating the comminuted plant raw materials with an acid, e.g., phosphoric or nitric acid, providing a pH in the range of pH 1-2.5. This treatment results in a suspension consisting of a solid phase containing cellulose components and a liquid phase containing dissolved pectin. The mixture is mashed, neutralised and finally dried to form pecto-cellulosic dry matter. The mashed mixture may also be separated into a solid and a liquid phase, which are neutralised individually and dried to give a pectin product and a pecto-cellulosic product.
In an alternative method as described in WO 05/003178 the plant material is used for providing an in situ system by swelling the plant material in a suspension comprising a salt which participates in keeping the structure of the plant material intact. When the plant material is in this swollen state the pectin can be chemically modified in a homogeneous manner. Subsequently, the modified pectin in combination with the remaining plant material may be worked up as a fibre containing pectin product or the pectin may be isolated providing a pectin product.
Thus, due to the highly industrial interests in the field of modifying pectin and in the production of different pectin products new methods are desirable for modifying and providing new pectin products.